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        // var arr1=[3,4,5];
        // var arr2=[6,7,8];
        // var res=arr.concat([arr1],[arr2]);
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        // var arr=[[1,2,3]];
        // var arr1=[3,4,5];
        // var arr2=[6,7,8];
        // var res=arr.concat([arr1],[arr2]);
        // console.log(res);
        var arr=[1,2,3];
        var arr1=[3,4,5];
        var arr2=[6,7,8];
        var res=arr.concat(arr1,arr2);
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